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\begin{document}

\title{Limited evidence of vertical root segregation in a subtropical forest}



\author[1]{Wenqi Luo}%
\author[2]{Ming Ni}%
\author[3]{Youshi Wang}%
\author[3]{Run Lan}%
\author[4]{David Eissenstat}%
\author[5]{James Cahill}%
\author[6]{Buhang Li}%
\author[7]{Chengjin Chu}%
\affil[1]{State Key Laboratory of Biocontrol and School of Ecology, Sun Yat-sen University, 510275 Guangzhou, China }%
\affil[2]{Universite de Sherbrooke}%
\affil[3]{Affiliation not available}%
\affil[4]{Pennsylvania State University}%
\affil[5]{University of Alberta}%
\affil[6]{Sun Yat-sen University}%
\affil[7]{Sun Yat-Sen University}%


\vspace{-1em}



  
  \date{May 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Vertical root segregation can be a key underpinning of species
co-existence through below-ground niche partitioning but has rarely been
tested in diverse forest communities. We randomly sampled \textgreater{}
4000 root samples from 625 0-30 cm soil profiles in a subtropical forest
in China to determine the degree of vertical root segregation among 109
woody species and rooting plasticity in response to edaphic
heterogeneity and root neighbours. Over 85\% of species were
predominantly distributed in the 0-10 cm soil zone, exhibiting low and
inconsistent rooting plasticity in response to either edaphic
heterogeneity or root neighbours. There was no evidence of vertical root
segregation among co-occurring species. Contrastingly, the increase of
one species' root abundance tended to increase, but not reduce other
species' root abundance within soil zones. These findings suggest that
interspecific differentiation of resource acquisition strategies might
be more important than root segregation in mediating species
co-existence in diverse forests.%
\end{abstract}%



\sloppy


\textbf{Title:} Limited evidence of vertical root segregation in a
subtropical forest

\textbf{Running title:} Rooting patterns of subtropical trees\textbf{Key
words:} DNA barcoding, rooting plasticity, subtropical forest, vertical
root segregation

\textbf{Type of article:} Letters\textbf{Number of words in the
summary:} 150\textbf{Number of words in the main text:}
5021\textbf{Number of references:} 61\textbf{Number of figures:} 5

Wenqi Luo\textsuperscript{1,2}, Ming Ni\textsuperscript{1,4}, Youshi
Wang\textsuperscript{1}, Runxuan Lan\textsuperscript{1}, David M.
Eissenstat\textsuperscript{2}, James Cahill\textsuperscript{3}, Buhang
Li\textsuperscript{1}, Chengjin Chu\textsuperscript{1*}

\textsuperscript{1}State Key Laboratory of Biocontrol and School of
Ecology, Sun Yat-sen University, 510275 Guangzhou, China

\textsuperscript{2}Department of Ecosystem Science and Management, The
Pennsylvania State University, University Park, PA 16802, USA

\textsuperscript{3}Department of Biological Sciences, University of
Alberta, Edmonton, AB T6G 2E9, Canada

\textsuperscript{4}D\selectlanguage{ngerman}épartement de Biologie, Université de Sherbrooke,
Sherbrooke, QC, Canada\textbf{Corresponding author:} Chengjin Chu
(\emph{chuchjin@mail.sysu.edu.cn}). Tel: (+86) 2084111541. Mailing
address: Xingang Xi Road 135, Sun Yat-sen University, Guangzhou, 510275,
China\textbf{Statement of authorship:} WL and CC conceived the project.
WL synthesized the data and conducted the analyses with inputs from MN
and JC. WL wrote the manuscript with input from CC, JC, YW and D.M.E.
WL, RL and BL collected the root and soil data.\textbf{Data
accessibility statement:} The data supporting the results will be
archived in Dryad and the data DOI will be included once the manuscript
is accepted.

\textbf{Summary}

Vertical root segregation can be a key underpinning of species
co-existence through below-ground niche partitioning but has rarely been
tested in diverse forest communities. We randomly sampled \textgreater{}
4000 root samples from 625 0-30 cm soil profiles in a subtropical forest
in China to determine the degree of vertical root segregation among 109
woody species and rooting plasticity in response to edaphic
heterogeneity and root neighbours. Over 85\% of species were
predominantly distributed in the 0-10 cm soil zone, exhibiting low and
inconsistent rooting plasticity in response to either edaphic
heterogeneity or root neighbours. There was no evidence of vertical root
segregation among co-occurring species. Contrastingly, the increase of
one species' root abundance tended to increase, but not reduce other
species' root abundance within soil zones. These findings suggest that
interspecific differentiation of resource acquisition strategies might
be more important than root segregation in mediating species
co-existence in diverse forests.

\textbf{Introduction}

Fine roots (\textless{}2 mm) are the most metabolically active part of
root systems responsible for soil nutrient uptake, mycorrhizal symbiosis
and interspecific interactions (Valverde-Barrantes \emph{et al.} 2015;
Eissenstat \emph{et al.} 2015). In most ecosystems, fine roots are
concentrated within the upper soil surface (e.g. 0-20 cm) where there
are typically higher nutrient availability and lower physical impedance
(Schenk \& Jackson 2002). However, even within such a shallow profile,
species may differ in their vertical relative root distributions, which
is defined as the proportions of roots partitioned among soil zones
(Herben \emph{et al.} 2018; Zhou \emph{et al.} 2020). Such difference
may promote species co-existence through a more complete filling of
below-ground space (Ma \& Chen 2017) and complementary resource use
among species (Blume\selectlanguage{english}-Werry \emph{et al} . 2019). Nevertheless, very few
studies have examined species-specific vertical root distributions in
diverse forest communities, primarily due to the difficulty in
identifying root materials to species. The lack of such information has
hindered our understanding of rooting-depth mediated below-ground niche
differentiation, root foraging strategies and species spatial
distributions (Gale \& Grigal 1987; Klime\selectlanguage{polish}š\selectlanguage{english}ov\selectlanguage{ngerman}á \emph{et al.} 2016).

As root systems are highly plastic organs, vertical root distributions
could express high phenotypic plasticity in response to external factors
such as edaphic heterogeneity (Sainju \& Good 1993; Case \emph{et
al.}2020) and specific root neighbours (Zhang \emph{et al.} 2019;
Chen\emph{et al.} 2020). Such adjustments may increase root nutrient
uptake efficiency through optimizing root placement according to
nutrient availability and neighbouring root identities (Mommer \emph{et
al.}2012; Zhang \emph{et al.} 2019). While effort is increasingly
devoted to elucidating the mechanisms and consequences of root
segregation (Holdo 2013; Kulmatiski \emph{et al.} 2017), most studies
are biased to artificial communities (Mahall \& Callaway 1992; Belter \&
Cahill 2015; Zhang \emph{et al.} 2019) or natural communities in arid
and semi-arid regions, typically involving two species (Case \emph{et
al.} 2020; Chen\emph{et al.} 2020; but see Valverde-Barrantes \emph{et
al.} 2015). As the direction and extent of rooting plasticity may be
highly dependent on the number and identities of root neighbours, it
remains uncertain if and to what extent relative root distributions
would be affected by the presence of multiple interacting neighbours in
complex forests. Such information is critical for the prediction of how
the change of species composition and edaphic conditions may influence
competitive interactions and root neighbourhood stability (Chen \emph{et
al.} 2018a, b).

Within soil zones, the degree of interspecific root segregation could be
ultimately determined by their innate difference in root system
architecture (Schenk \& Jackson 2005) and rooting plasticity in response
to edaphic heterogeneity or/and neighbour structure (Zhang \emph{et
al.}2019; Case \emph{et al.} 2020; Zhou \emph{et al.} 2020). Despite the
proposed mechanisms, the generalization of vertical root segregation in
natural communities have not been fully identified, with both apparent
(Kesanakurti \emph{et al.} 2011; Herben \emph{et al.} 2018) and minor
rooting differentiation observed (von Felten \& Bernhard 2008;
Hoekstra\emph{et al.} 2015). Whether and how rooting differentiation
mediates species co-existence remains a subject of intensive studies.
Valverde-Barrantes~\emph{et al.} ~(2015) recently reported that root
depths of temperate trees in the 0-60 cm soil zone were largely
overlapped and exhibited low responsiveness to neighbours. However, root
depths alone may fail to fully capture root niches, as species with
similar root depths may differ considerably in their relative root
distributions (Schmid \& Kazda 2001; Bennett \emph{et al.} 2002;
Herben\emph{et al.} 2018). It is thus unclear whether relative root
distribution represents a root niche dimension independent of root depth
that may contribute to vertical root segregation.

In this study, we collected \textgreater{} 4000 root segments from 625
soil samples within the 0-30 cm soil zone in a subtropical forest,
China. From these samples, we quantified the relative root distributions
of 53 woody species, modeled rooting plasticity of the 29 most common
species as a function of soil heterogeneity and root neighbours and
finally examined how individual roots of the identified 109 species were
vertically placed. We used these data to test three hypotheses. First,
we predicted high interspecific differentiation in relative root
distributions, with some predominantly placing their roots in specific
soil zones whereas some others being homogeneously distributed. Second,
we predicted high rooting plasticity in response to either edaphic
heterogeneity or/and root neighbours that may optimize root placement
according to nutrient availability and neighbour root competition.
Specifically, due to the potential pre-emptive utilization of soil
resources by some proliferative species (Campbell \emph{et al.} 1991),
we predicted that the increase of one species' root abundance would
reduce other species' root proliferation, resulting in pervasive
negative correlations in relative root abundance among species pairs.
Finally, depending on the differentiation in relative root distributions
among species complementarily combined with high rooting plasticity, we
predicted that heterospecific roots would be vertically segregated
either simply by avoiding co-occurring or by avoiding placing similar
relative root abundance in the same soil zones.

\textbf{Materials and Methods}

\textbf{Site description}

This study was conducted in the 50-ha Heishiding (HSD) Forest Dynamic
Plot (23.455 °N, 111.908 °E; altitude: 435-698 m) in Guangdong province,
China. This area belongs to subtropical monsoon climate, with a mean
annual temperature of 19.6 °C and mean precipitation of 1740 mm. The
plot has a rough terrain with the slope ranging from 7 ° to 75 °. The
floristic composition of the plot is characteristic of evergreen
broad-leaved forests with \emph{Castanopsis carlesii}
(Fagaceae),\emph{Cryptocarya concinna} (Lauraceae) and \emph{Altingia
chinensis}(Altingiaceae) being the dominant canopy species.

\textbf{Root excavating and identification}

Root sampling was conducted in early summer from April to June in 2016
by randomly selecting 625 locations in the 50-ha plot as described in
detail in Luo \emph{et al.} (2020). In brief, at each location, three
300-cm\textsuperscript{3} soil cubes (5 × 6 × 10 cm) were sequentially
extracted with a depth increment of 10 cm using a 5 × 6 × 40 cm steel
spade. To further explore vertical root distributions in deeper soils,
12 cubes were sampled to a depth of 100 cm. An additional
300-cm\textsuperscript{3} soil cube of soil next to each root-sampling
location was collected for further analyses of soil chemical and
textural properties.

In the laboratory, each soil cube was soaked in water for \emph{c} . 40
min and intact and all well-branched woody fine root segments (\selectlanguage{english}[?] 2 mm in
diameter) with length \selectlanguage{english}[?] 10 cm were selected for further analysis. Root
segments generally included 2-4 orders of roots
(Valverde\selectlanguage{english}-Barrantes\emph{et al.} 2015). For each soil cube, the selected
root segments were further sorted into morphotypes based on root
branching patterns, color, distinctive odor, diameter and the presence
of nodules or latex (Luo\emph{et al} . 2020). As reported in Luo
\emph{et al.} (2020), some root morphotypes could be confirmed to
species based on their identifiable and conservative root
characteristics. All other morphotypes were sequenced and compared with
four DNA barcodes (\emph{rbcLa} ,\emph{ITS2} , \emph{matK} and
\emph{trnH-psb} ) generated from leaf collections of 237 tree canopy
species in the plot (Zhou 2014). Following these procedures, we
identified \emph{c} . 94\% of the morphotypes that in combination
contained 143 species. 109 species with occurrences \selectlanguage{english}[?] 5 cubes were
selected for the present study.

\textbf{Soil variables}

Thirty-two soil variables were measured for each sampling location,
including a number of macro- and micronutrients and variables associated
with soil textural properties (Table S1). Details about soil variables
measurements are described in Table S2. Preliminary analysis showed that
most micronutrients, excepting for soil copper (Cu), had weak
associations with fine root trait plasticity and biomass accumulation.
Consequently, we selected 13 soil variables previously identified in the
literature (Kesanakurti \emph{et al.} 2011; Valverde-Barrantes \emph{et
al.} 2015a; Luo \emph{et al.} 2020) to frequently influence root growth
and trait variation for the present study, including soil pH, electrical
conductivity, total organic carbon, total nitrogen, total phosphorus,
total potassium, hydrolysable nitrogen, available phosphorus, available
potassium, sand content, silt content, clay content and available Cu.

\textbf{Quantification of species-level root-placement patterns and
relative root distributions}

Species root-placement patterns were categorized into seven types based
on their occurrences in the 0-30 cm soil zone (Fig. 1). These included
roots that occurred only in the 0-10 cm soil zone (type 1), 10-20 cm
soil zone (type 2), 20-30 cm soil zone (type 3), throughout the 0-20 cm
soil zone (type 4), throughout the 0-30 cm soil zone (type 5),
throughout the 10-30 cm soil zone (type 6) and roots that occurred both
in the 0-10 and 20-30 cm soil zone (type 7). For a soil cube, individual
species' root-placement pattern was assigned with one of the seven
possible rooting types (Fig. 1). Relative root distributions were
determined as the relative root abundance in the 0-10, 10-20 and 20-30
cm soil zones, with zones without roots assigned a score of 0\%. Root
abundance was indicated by species fine root length because root length
is likely a better indicator of foraging effort than root mass. As
individual root segments could not be accurately traced to specific
parental trees, we focused on species-level relative root distribution
for the 109 species with occurrences \selectlanguage{english}[?] 5 cubes. Each 30-cm deep soil
cube represents a root neighbourhood with three soil zones.

\textbf{Statistical analysis}

\textbf{Testing the differentiation of relative root distributions among
species}

To increase statistical power, the comparisons of relative root
distributions were focused on the 53 most common species with
occurrences \selectlanguage{english}[?] 15 cubes. For a given species, we calculated the averaged
value of relative root abundance in each soil zone of that species and
tested whether soil zone significantly influenced the means of relative
root abundance using a one-way Welch's ANOVA. Pairwise comparisons of
the means of relative root abundance among soil zones was performed
using the Games-Howell Post-Hoc Test. Finally, we categorized the 53
species into different groups based on their relative root distributions
using clustering analysis.

\textbf{Testing rooting plasticity in response to edaphic heterogeneity
and root neighbours}

The plasticity of relative root distributions in response to edaphic
heterogeneity and root neighbours was evaluated for each species with
separate redundancy analysis (RDA). As it is required in an RDA analysis
the number of observations of the response variable should be greater
than that of explanatory variables to avoid overfitting
(Legendre\emph{et al.} 2013), we focused on the 29 most common species
with occurrences \selectlanguage{english}[?] 30 cubes. This maximized the number of root
neighbours that could be included in the RDA analysis, giving the
minimum observations of 30 in the relative root distribution for the
focal species.

For each species, relative root abundance (\%) in the 0-10 cm soil zone
was treated as the response variable. Additionally, two predictor
variable matrices were created, one of which described the
presence-absence of the remaining 28 species within the 0-30 cm soil
zone and the other described edaphic conditions regarding the 13
selected soil variables. We then performed a global analysis to test the
importance of each factor in the matrix separately (ter Braak \emph{et
al.} 2012).When the global analysis was significant, a step-wise RDA was
performed to select the parsimonious subsets of competitor species and
soil factors as explanatory variables (Blanchet \emph{et al.} 2008).
Lastly, we performed a variance partitioning to determine the relative
importance of soil variables and root neighbours on rooting plasticity
as described in Peres-Neto \emph{et} \emph{al.} (2006).

To examine whether the increase of one species' root abundance would
reduce neighbours' root abundance, we performed a pairwise correlation
of relative root abundance between a focal species and each of the
remaining 28 species. Finally, to examine whether species relative root
distributions would be influenced by heterospecific root crowdedness, we
used a correlation analysis to examine the relationship between the
focal species relative root distributions and both heterospecific root
richness and total root length in root neighbourhoods.

\textbf{Testing vertical root segregation in root neighbourhoods}

Vertical root segregation was tested in two complementary ways using all
the identified root morphotypes (i.e. a specific species composed of
several root segments) belonging to 109 species sampled from 604 root
neighbourhoods. First, we tested whether species were vertically
segregated simply by exhibiting different root-placement patterns (i.e.
root depth) within the 0-30 cm soil zones (Fig. 1a, 1c). Second, we
sequentially tested whether species were vertically segregated by
avoiding placing similar relative root abundance within the 0-10, 10-20
and 20-30 cm soil zone, respectively (Fig. 1b, 1d).

As each root morphotype was assigned a specific rooting type (Fig. 1)
that was treated as a categorical functional trait, we constructed a
trait dendrogram depicting the pairwise functional distance of all the
identified and coded morphotypes based on a Gower Distance and
determined the mean pairwise functional distances (MPD) of root
neighbourhoods (Gower 1971; Laliberte \emph{et al.} 2014; Swenson 2014).
We then constructed 999 null communities for each root neighbourhood,
from which the mean and the standard deviations of the 999 simulated
MPDs were determined using the picante package (Kembel \emph{et
al.}2010). Null communities were constructed by randomizing the pairwise
functional distances of the coded morphotypes by shuffling the tip names
of the trait dendrogram, keeping species occurrences constant. Finally,
we determined the standardized effect size of the functional MPD
(SESMPD) to quantify the functional similarity of co-occurring species
based on their root-placement patterns following the equation:

SESMPD =\(\frac{(\text{MPD}_{\text{obs}}\ -\ mean\ (\text{MPD}_{\text{null}}))}{sd\ (\text{MPD}_{\text{null}})}\),

where MPD\textsubscript{obs} represents the observed MPD. The mean
(MPD\textsubscript{null}) and sd (MPD\textsubscript{null}) represent the
mean and the standard deviations of the 999 simulated MPDs,
respectively. Root neighbourhoods with a positive SESMPD indicated
functional overdispersion (i.e. vertical root segregation) with values
greater than 1.96 indicating significant segregation, whereas root
neighbourhoods with a negative SESMPD indicated functional clustering
(i.e. vertical root aggregation) with values below -1.96 indicating
significant root aggregation (e.g. Swenson 2014). The absolute value of
SESMPD indicated the magnitude of the difference between the observed
and the simulated MPD. Similarly, we quantified root segregation based
on species relative root distributions simply by replacing the
root-placement-pattern-based trait dendrogram with a trait dendrogram
constructed based on species relative root distribution. In addition to
using MPD, we also used the standard deviation and variance of relative
root distributions to evaluate the similarity of root depths among
co-occurring species. For a specific soil zone, we constructed a trait
dendrogram based on species relative root abundance in the soil zone.

Under the prediction of vertical root segregation, either by way of
avoiding co-occurring or placing similar relative root abundance within
the same soil zone, we would expect pervasive functional overdispersion
in root neighbourhoods, which could be supported by the following two
observations. First, there existed a higher proportion of root
neighbourhoods that exhibited functional overdispersion than clustering.
Second, and more importantly, that the difference between the observed
and simulated MPD, measured as the mean of the SESMPD, was significantly
higher than the expectation of zero when using a two-tail
Student's\emph{t} test (Yang \emph{et al.} 2014).

All statistical analyses were performed in the R 3.6.0 statistical
software (R Development Core Team, 2019).

\textbf{Results}

\textbf{Differentiation of relative root distributions among species}

At the root neighbourhood level, root species richness per sampling unit
(5x6 cm) in the 0-10, 10-20 and 20-30 cm soil zone was 4.15, 2.73 and
2.18, respectively (Figs. 2a, S1a) and decreased to approximately one
below 40 cm (Fig. 2a). Root abundance also decreased sharply as
increasing soil depths with over 65\% of roots concentrating within the
0-30 cm soil zone (Figs. 2b, S1b).

In support of our hypothesis, although most species were found in the
0-10 cm soil zone (Figs. 3a, S2), species differed in their relative
root distributions that could broadly be categorized into five groups
(Table S3; Fig. 3b). Specifically, for over 50\% of species, root
abundance was the highest in the 0-10 cm soil zone and had similar root
abundances in 10-20 and 20-30 cm soil zone (group A). For 21\% of
species, root abundance was the highest in the 0-10 cm soil zone and
decreased significantly toward the deeper zones (group B). 11\% of
species were homogeneously distributed throughout the 0-30 cm soil zone
(group C; Fig. 3b). An additional 18.8\% of species similarly had the
highest fraction of roots in the 0-10 cm soil zone with their root
abundance exhibiting inconsistent patterns among the deeper two zones
(group D). For some species, root abundance in the 20-30 cm zone was
significantly higher than the 10-20 cm zone or that root abundance was
similar among the uppermost two zones. One species' roots were
concentrated in the 10-20 cm soil zone (\emph{Xanthophyllum hainanense}
, group E).

\textbf{Vertical root segregation in root neighbourhoods}

Our hypothesis that individual species in root neighbourhoods would be
vertically segregated was generally not supported. Although there were
higher proportions of root neighbourhoods where heterospecific roots
tended to avoid co-occurring (Fig. 4a) or avoid placing similar root
abundance in the same soil zone (Table 1; Fig. 4b-d) than that tended to
co-occur, the proportions of root neighbourhoods that either exhibited
significant root segregation or aggregation were very low and did not
differ substantially (Table 1). Furthermore, the mean of the
standardized effect size of the four parameters measuring rooting
similarity did not differ significantly from zero (Fig. 4), indicating
that the diversity of root-placement patterns in the observed
communities did not differ significantly from that of randomly simulated
distributions. Moreover, in contrast to our prediction, when only roots
that occurred throughout the 0-30 cm soil zone were included in the
analysis, we found a higher proportion of root neighbourhoods that
exhibited root aggregation (54\%-76\%) than that exhibited root
segregation (24\%-47\%; Table S4). Such aggregated patterns were
significant in the 20-30 cm soil zone when rooting similarity was
evaluated based on the variance of co-occurring species' relative root
distribution (Table S4), suggesting that species have similar root
abundance in the 20-30 cm soil zone.

\textbf{Rooting plasticity in response to edaphic heterogeneity and root
neighbours}

Our hypothesis that relative root distribution would express high
plasticity in response to edaphic heterogeneity or/and root neighbours
was not fully supported. For the 29 most common species, we only found
two species' relative root abundance in the 0-10 cm zone to be
influenced by the presence of the remaining 28 species in the 0-30 cm
soil zone (Fig. S3). When interspecific root interactions were examined
by correlating pairwise species' relative root abundance of the 29
species in the 0-10, 10-20 and 20-30 cm soil zone, there were 7.6\%, 4.9
\%, and 7.1\% of species pairs that exhibited a significant correlation,
respectively (Fig. 5). However, contrary to our prediction, excepting
for some negative correlations observed in the 0-10 cm soil zone (Fig.
5a), all the correlations were positive (Fig. 5b, 5c), suggesting that,
if such interactions were direct, the increase of one species' root
abundance tends to increase, but not overwhelm other species' root
abundance (Fig. 5). Again, contrary to our hypothesis, species' relative
root abundance in the 0-10 cm zone was neither affected by
heterospecific root richness (Fig. S4) nor by root abundance (i.e. total
root length) in the 0-30 cm soil zone (Table S5).

When heterospecific root richness and abundance were quantified only for
the 0-10 cm soil zone, only two of the six most common species exhibited
significant rooting plasticity in response to neigbours with contrasting
responses. While~\emph{Schima superba} tended to place fewer roots in
the 0-10 cm soil zone with increased heterospecific root richness (Table
S6), \emph{Altingia chinensis} placed more roots in the 0-10 cm soil
zone with increased heterospecific root abundance (Table S7), suggesting
a relatively minor and inconsistent effect of heterospecific root
crowdedness on the focal species relative root abundance. Similarly,
there were only four of 29 species whose relative root abundance
exhibited significant plasticity in response to edaphic heterogeneity
(Fig. S3). For all the four species, relative root abundance in the 0-10
cm soil zone was negatively correlated with soil nutrient availability,
suggesting increased root proliferation in nutrient-poor soil patches.

\textbf{Discussion}

\textbf{Root neighbourhood level and species-level relative root
distribution}

Examination of relative root distributions revealed the concentration of
fine roots in the 0-10 cm soil zone in our experimental subtropical
forest. Both root richness and abundance were largely reduced below 40
cm (Fig. 2). Similarly, individual species' root abundance declined
sharply and became variable in the deeper zones, presumably due to
higher edaphic heterogeneity (Fig. S5). Consequently, although we
examined root distributions only within the 0-30 cm soil zone, this
allowed the examination of \emph{c.~} 70\% of fine roots' placement
patterns and interspecific interactions across the 0-100 cm soil profile
(Fig. 2b).

Our study partially supported the differentiation of relative root
distributions among species. Although a majority of species had the
highest root abundance in the 0-10 cm soil zone, some canopy trees
predominantly placed their roots in the 10-20 cm soil zone or were
homogeneously distributed throughout the 0-30 cm soil zone (Fig. 3b).
These findings expand prior studies indicating similar root depths of
coexisting temperate trees (Valverde-Barrantes~\emph{et al.} 2015),
raising the possibility of interspecific vertical root segregation
through differentiation in relative root distributions (Schmid \& Kazda
2001; Herben \emph{et al.} 2018).

\textbf{Plasticity of relative root distribution in response to root
neighbours and edaphic heterogeneity~}

Our hypothesis that relative root distribution would exhibit high
plasticity in response to edaphic heterogeneity and root neighbours was
not fully supported. For most species, relative root distribution was
neither affected by edaphic heterogeneity nor by the presence of
specific root neighbours (Fig. S3), suggesting that neighbourhood
species composition is not a good predictor of a focal species' root
depth. Likewise, most species' relative root distributions were
unaffected by heterospecific root richness or abundance (Tables S5-S7;
Figs. S3, S4), as observed in temperate forests and mountain grasslands
(Valverde-Barrantes \emph{et al.} 2015; Herben \emph{et al.} 2018).
Similarly, Cahill (2003) reported that grassland species did not exhibit
substantial changes in root-shoot ratios with increasing below-ground
competition, suggesting that increased heterospecific root mass may not
necessarily be indicative of higher competitive pressure exerted on the
focal species. However, these findings contrasted with previous studies
reporting apparent neighbour-induced rooting plasticity (e.g.
Cahill\emph{et al.} 2010; Belter \& Cahill 2015; Zhang \emph{et al.}
2019). An explanation for the discrepancy might be that, in contrast to
greenhouse experiments that have usually involved two species (Cahill
\emph{et al.}2010; Belter \& Cahill 2015), individual species in natural
communities experience high niche overlap and diffuse competition.
Specifically, in competitive neighbourhoods composed of multiple
species, the addition or removal of a single neighbour may not have a
significant effect on the competition experienced by the target plant
(Dyer \& Rice 1997). Another reason might be that in species-rich
communities, species exhibit varied rooting plasticity,
both~directionally~and~magnitudinally, in response to different
neighbours, resulting in overall neutral adjustments of root depth
(Litav 1967; Brisson \& Reynolds 1994; Caldwell \emph{et al.}1996;
Schenk \emph{et al.} 1999).

Root proliferation usually decreases as increasing heterospecific root
abundance, presumably due to intensified competitive pressure and
stronger chemical inhibitive effects (Brisson \& Reynolds 1994).
However, we found that the increase of one species' root abundance did
not reduce other species' root abundance (Fig. 5). Instead, the
pervasive positive associations of species pairs suggest concurrent root
proliferation of co-occurring species into the same soil zones (Fig. 5b,
5c; Belter \& Cahill 2015). Nevertheless, based on the positive
association among two species, we could not conclude that such species
can mutually and directly facilitate their root growth, as such
correlations could simply result from the collinearity of species root
abundance with other unaccounted-for factors. For instance, similar
responses of root proliferation to limiting nutrients or soil microbes
may lead to the co-occurrence of the paired species (Rog \emph{et
al.}2020). These uncertainties highlight the need for further
manipulative experiments examining the direct and indirect paths of
which root-to-root interactions play to better understand below-ground
processes in complex forest communities (Belter \& Cahill 2015;
Zhang\emph{et al.} 2019).

At the species level, the variation of relative root abundance in the
0-10 cm soil zone was high (CV: 38\%-144\%), suggesting that, in
addition to the possible effects of edaphic heterogeneity and root
neighbours, other factors also contribute to such variations. For
instance, as root segments were randomly sampled, those sampled on the
margin of the root system could be relatively shallowly placed, giving
that root abundance generally declines with increasing distance from the
parental tree stem (Jones \emph{et al.} 2011). Such spatial arrangement
of root systems could cause additional rooting variations independent of
soil heterogeneity and root neighbours. However, prior studies also
indicated that root-placement patterns were mainly affected by localized
edaphic conditions, as root systems are modular, and each meristem can
generate a potentially flexible and plastic root branches
(Waisel\emph{et al.} 2002). Nevertheless, the change of whole-plant root
system arrangements still represents adaptive plasticity in response to
root neighbours or soil conditions (Casper \emph{et al.} 2003).
Collectively, although there were substantial variations in relative
root distribution left unexplained that could be attributable to tree
ontogenetic stages or soil microbial interactions, our study suggests
the relatively minor effect of interspecific interactions on a focal
species' root distribution.

\textbf{Limited evidence of vertical root segregation in root
neighbourhoods}

Our hypothesis that the differentiation in relative root distribution
among species, complementarily combined with substantial rooting
plasticity in response to edaphic conditions and neighbours, would
ultimately result in vertical root segregation was not supported (Fig.
4). By contrast, the low differentiation of relative root distribution
(Fig. 3), the neutral effect of root-to-root interactions on species
rooting plasticity (Figs. 5, S3) and the random-to-aggregated
root-placement patterns (Fig. 4; Table S4) suggest similar topsoil
foraging of heterospecific roots. The generally random root-placement
pattern observed here was similar to that reported in grasslands
(Frank\emph{et al.} 2010) and greenhouse experiments (Litav 1967;
Semchenko\emph{et al} . 2007). These findings were consistent with the
`symmetric root proliferation' hypothesis in that heterospecific roots
can access and proliferate into the same soil patch (Valverde-Barrantes
\emph{et al.} 2015). Potential explanations for the lack of strong root
segregation are elaborated below.

First, the differentiation of relative root distribution maybe not high
enough to allow species to exhibit distinctive root depths specialized
into specific soil zones. For instance, among the 53 species, only one
species had roots concentrating within the 10-20 cm soil zone, and no
species were concentrated in the 20-30 cm soil zone (Fig. 3b). Although
some species may have higher root abundance in the deeper zones, based
on neighbourhood- and species-level relative root distributions, we
think it unlikely. The accumulation of \emph{c} .70 \% of fine roots in
the 0-30 cm soil zone (Fig. 2b) and the co-occurrence of multiple
species in the deeper zones (Fig. S5) suggest fairly shallow and
homogeneous fine root distributions in the plot. Additionally, as
revealed by the Post-Hoc comparison of species relative root abundance
in the 10-20 cm soil zone, there were only seven of 2703 possible
combinations of species pairs that exhibited significant rooting
difference and all were found between~\emph{X. hainanense} ~and the
other seven species, further suggesting minor interspecific differences
in root depth. Moreover, although \emph{X. hainanense} ~had distinctive
root depth, most of its individuals were\emph{~} aggregated into the
valley of the plot, rather than being widely distributed that would
otherwise increase the diversity of root-placement patterns in root
neighbourhoods. By contrast, the two most common species (\emph{Pinus
massoniana} ~and~\emph{Altingia chinensis} ) were widely distributed
across the plot, sharing similar rooting patterns with multiple
co-occurring species (i.e. concentrating in the 0-10 cm soil zone; Fig.
3b), potentially resulting in spatial root aggregation.

Second, the symmetric root competition may also explain the lack of
vertical root segregation. It is proposed that a dominant species with
large root biomass does not get disproportionate fitness advantages, as
an increase of neighbours' root biomass, even minor, may similarly exert
competitive pressure on the dominant species' fitness (Cahill \& Casper
2000; Demalach \emph{et al.} 2016). The underlying mechanism may be that
species competitive abilities cannot be ranked along a hierarchy in
which only a single species gains competitive dominance (Gilpin 1975;
Buss 1980). Instead, in an intransitive competitive network, when for
example species A is superior to species B, and B superior to C, it is
more likely that C is superior to A (A\textgreater{}B; B\textgreater{}C;
C\textgreater{}A; Buss, 1980). The correlations of species root biomass
also imply that both facilitative and inhibitive interactions exist
among species (Fig. 5a). Such an intransitive competitive network may
prevent the predominance of a single species in root neighbourhoods
(Laird \& Schamp 2006).

Third, as found in our recent study in the same plot and elsewhere
(Weemstra \emph{et al.} 2016; Luo \emph{et al.} 2020), the
multidimensional nature of roots may also explain the prevalence of
symmetric root proliferation. This suggests that species with different
trait syndromes may exhibit comparable competitiveness under similar
edaphic conditions. For instance, although thin roots with larger
specific root surface area appear to be more competitive in nutrient
foraging, due to their higher mycorrhizal colonization rates, thick
roots may have larger hyphae surface area than thin roots. As a result,
for a given amount of carbon for mycorrhizal root construction, thick
and thin roots could have comparable absorptive areas and thus foraging
capacities (Kong \emph{et al.} 2014). This also helps explain the
widespread co-existence of species with contrasting root morphologies
and mycorrhizal types in temperate forests (Chen \emph{et al.} 2018b;
Valverde-Barrantes \emph{et al.} 2018). Lastly, it is noteworthy that
optional mechanisms rather than vertical root segregation also help to
promote species co-existence. For instance, despite the similar root
abundance partitioned into a specific soil zone, species may differ in
the amount of biomass allocated to thin roots that may cause
differentiation of root functional traits among species (Mommer \emph{et
al.} 2011). Such alternative strategies may confer a similar competitive
advantage in response to edaphic heterogeneity and neighbour root
competition.

\textbf{Conclusions}

Our results found that over 85\% of species predominantly placed their
fine roots in the 0-10 cm soil zone, resulting in the accumulation
of\emph{c.} 70\% of fine root abundance in the 0-30 cm soil zone in our
experimental subtropical forest. The low differentiation in relative
root distribution among species, the neutral effect of root neighbours
on species relative root distributions and the random-to-aggregated
root-placement patterns suggest similar topsoil foraging among
heterospecific roots and the adoption of alternative resource
acquisition strategies that promotes species co-existence.

\textbf{Acknowledgments}

This research was financially supported by the National Natural Science
Foundation of China (31925027, 31622014 and 31570426 to CC) and the
Fundamental Research Funds for the Central Universities (20lgpy116 to
YW). DME acknowledges partial support from USDA NIFA Project \#PEN04744
and Accession \#1023222.

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\textbf{Table 1} Testing interspecific vertical root segregation either
based on root-placement patterns (MPD\textsubscript{placements})
throughout the 0-30 cm soil zone or relative root abundance
(MPD\textsubscript{abundance}) in the 0-10 (P1), 10-20 (P2) and 20-30 cm
(P3) soil zones using three different parameters. The similarity in
relative root abundance of co-occurring species was evaluated with mean
pairwise distances (MPD), standard deviation (SD) and variance
(Variance) of species relative root abundance. Values indicate the
proportions of root neighbourhoods with root segregation (Sepa.),
significant segregation (Sig-S.), aggregation (Aggr.) and significant
aggregation (Sig-A.). The mean of the standardized effect size of the
corresponding parameter was compared with the null expectation of zero
to examine whether it was significantly differed from zero across the
entire study site.\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
\begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Parameter}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Soil} \textbf{zone}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Sepa.} \textbf{(\%)}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Aggr.} \textbf{(\%)}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Sig-S.} \textbf{(\%)}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Sig-A. (\%)}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
\textbf{Mean}\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
P\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
MPD\textsubscript{placement}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P1-P3\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
62\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
38\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.50\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.49\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.04\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.30\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
MPD\textsubscript{abundance}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P1\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
61\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
39\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.83\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.65\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.03\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.38\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
52\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
48\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.66\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.82\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.02\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.56\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P3\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
48\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
52\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.65\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.82\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.01\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.79\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
SD\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P1\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
61\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
39\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.98\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.32\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.04\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.35\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
54\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
46\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.83\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.65\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.02\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.57\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P3\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
49\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
51\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
2.31\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.66\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.01\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.77\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Variance\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P1\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
54\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
46\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.82\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.65\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.02\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.58\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
48\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
52\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.99\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
1.82\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.04\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.37\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
P3\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
44\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
56\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
2.31\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.83\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
-0.02\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
0.60\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{Figure legends}

\textbf{Figure 1~} Schematic diagram showing vertical root-placement
patterns of individual species within the 0-30 cm soil zone. Each number
in the box indicates a unique species with the solid black lines
indicating species-specific roots. For each soil cube, species
root-placement patterns were assigned with one of the seven possible
rooting types based on their root depths. In the scenario of vertical
root aggregation, species with similar root-placement patterns (a) or
relative root distributions (b) are more likely to co-occur. In the
scenario of vertical root segregation, species with different
root-placement patterns (c) or relative root distributions (d) are more
likely to co-occur. The width of the shapes in panel (b) and (d) is
proportional to the species' relative root abundance.

\textbf{Figure 2~} Root species richness (a) and relative root abundance
(b) along the 0-100 cm soil profile. Different letters indicate
significant difference among soil zones.

\textbf{Figure 3~} Relative root distributions for the 53 most common
species within the 0-30 cm soil zone. Panel (a) shows the frequencies of
species occurrences in the 0-10, 10-20 and 20-30 cm soil zones. Panel
(b) shows the frequency distribution of species relative root abundance
in the 0-10 cm soil zone. Species with asters suggest that soil zone had
a significant effect on their relative root distributions, based on
which the 53 species were grouped into five groups. Group A, relative
root abundance in the 0-10 cm soil zone was significantly higher than
the deeper two zones, while the deeper two showed no significant
difference; Group B, relative root abundance in the 0-10 cm zone was the
highest and it decreased significantly toward the deeper zones; Group C,
relative root abundance did not differ significantly among soil zones;
Group D, relative root abundance was the highest in the 0-10 cm soil
zone while for some species, relative root abundance in the 20-30 cm
zone was higher than the 10-20 cm zone; Group E, relative root abundance
was the highest in the 10-20 soil zone; ns, nonsignificant.
*\emph{P}\textless{} 0.1, **\emph{P} \textless{} 0.05, ***\emph{P}
\textless{} 0.01.

\textbf{Figure 4~} Vertical root segregation evaluated based on
root-placement patterns (a) within the 0-30 cm soil zone and relative
root distributions (b-d) in the 0-10 cm soil zone. The similarity in
relative root abundance of co-occurring species was evaluated with mean
pairwise functional distances (MPD; b), standard deviation (SD; c) and
variance (Variance; d). The histograms illustrate the frequency
distribution of the standardized effect size of the corresponding
parameters across 604 root neighbourhoods. In panel (a), a positive
value indicates that co-occurring species in the 0-30 cm soil zone
tended to avoid co-occurring in the same soil zone, whereas a negative
value indicates that co-occurring species were more likely to co-occur
in the same soil zone than expected. In panels (b)-(d), a positive value
indicates that co-occurring species tended to avoid placing similar
relative root abundance in the 0-10 cm soil zone, whereas a negative
value indicates that co-occurring species tended to place more similar
relative root abundance than expected in the 0-10 cm soil zone. The
dashed red line in each panel indicates the mean value of the
standardized effect size of the corresponding parameters.

\textbf{Figure 5~} Pairwise correlations of relative root abundance for
the 29 most common species in the 0-10 (a), 10-20 (b) and 20-30 cm (c)
soil zone. Black solid lines link two species with significant positive
correlations, whereas blue dashed lines link two species with
significant negative correlations. The size of the lines is proportional
to the strength of the correlations.

\textbf{Supporting Information}

\textbf{Figure S1} Root richness per cube (a) and relative root
distributions (mean \selectlanguage{ngerman}± SE) (b) for the 613 0-30 cm soil cubes.

\textbf{Figure S2} Proportions of each rooting type (i.e. Fig. 1 in the
main text) of the 53 most common species.

\textbf{Figure S3~} Multivariate variance partitioning for four
individual species' relative root distributions.

\textbf{Figure S4} The effect of heterospecific root richness in the
0-30 cm soil zone on the six most common species' relative root
abundance (mean ± SE) in the 0-10 cm soil zone.

\textbf{Figure S5} Species-specific root distributions for the seven
most common species with occurrences \selectlanguage{english}[?] 3 cubes throughout the 0-100 cm
soil zone.

\textbf{Table S1} Summary of the 32 soil variables measured.

\textbf{Table S2} The details of measurements of soil variables.

\textbf{Table S3} The comparison of relative root abundance among soil
zones for the 53 most common species.

\textbf{Table S4} Testing vertical root segregation based on species
relative root abundance in the 0-10 (P1), 10-20 (P2) and 20-30 cm (P3)
soil zone using three different parameters.

\textbf{Table S5} The relationship between relative root abundance of
the six most common species in the 0-10 cm soil zone and heterospecific
root abundance in the 0-30 cm soil zone.

\textbf{Table S6} The relationships between relative root abundance of
the six most common species in the 0-10 cm soil zone and heterospecific
root richness in the 0-30 cm soil zone.

\textbf{Table S7} The relationship between relative root abundance of
the six most common species in the 0-10 cm soil zone and heterospecific
root abundance in the 0-10 cm soil zone.

\textbf{Hosted file}

\verb`Luo et al._Figures.docx`  available at \url{https://authorea.com/users/470716/articles/562671-limited-evidence-of-vertical-root-segregation-in-a-subtropical-forest}

\selectlanguage{english}
\FloatBarrier
\end{document}

